Finished ophthalmic lenses, such as progressive addition lenses or graduated prescription lenses, have traditionally been manufactured through a process starting with a semi-finished lens blank that is molded or cast between glass or metal molds. The semi-finished lenses are typically formed of one of a variety of synthetic polymeric materials and are commonly provided with a progressive design, a base curve, and an add power portion of the lens molded on a first surface, e.g. a front surface, of the lens. The second, back surface of the lens may be plano or curved and requires additional surfacing in order to provide the lens with the desired optical power and/or thickness. An optical laboratory grinds the desired prescription into the back surface of the lens blank using a generator and then polishes the back surface to complete the grinding process.
Advances in digital or direct surfacing techniques and equipment now allow for what has been referred to as free-form manufacturing of lenses. In free-form manufacturing, a generator is employed to form or grind the prescription, e.g. the add power, the base curve, and/or the corridor position, into a back surface of a lens blank having a spherical front lens blank so as to form the finished lens. The generator may use a single point cutter to produce the desired lens surface or surfaces. The cutting can only be performed on the back of a semi-finished lens. Most free-form generators will have a second or even a third cutter that produces a very smooth surface, e.g. by employing a diamond tip cutter, after the initial rough or bulk cutter removes the bulk of the undesired lens material. The lens is then finished on a specialized lens polisher that uses conformable tools or soft laps that buff the ground surface of the lens without destroying the surface details of the lens surface, i.e. without destroying or modifying the add power, base curve, and/or corridor position of the finished lens. An exemplary cycle time for the formation of a free-form manufactured, finished lens is provided in FIG. 7.
Due, in part, to the various advantages offered by lenses formed using free-form manufacturing techniques, for example the ease of customization, optical labs employing free-form manufacturing techniques are experiencing significant commercial pressure to increase finished lens throughput. The free-form lens manufacturing strategy also allows a lower inventory carry for optical laboratories because the cylinder and add power are generated into the back of the lens requiring only single vision blanks in one diopter spread. However, due to the highly sophisticated nature, significant cost, and limited production of the digital or direct surfacing machinery employed in free-form manufacturing, optical labs are often limited in their ability to simply increase throughput by employing additional direct surfacing machinery. Hence, there is a need in the field to increase the throughput of the existing direct surfacing machinery employed in the free-form lens manufacturing process.